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Registered
Join Date: Nov 2000
Location: Dallas
Posts: 3,601
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Fuel Tank Pressure Question
I am in process of dropping the engine on my '88 Carrera. Several days ago I disconnected the fuel line coming out of the "top" of the fuel filter as well as the line at the back left of the driver side fuel rail (i think its the return line). Not surprisingly I got a small dribble of fuel so no big deal. Then this morning I noticed some fuel that had overflowed out of the filter top and run down into engine compartment. I was trying to figure out WHY this would only happen a week after it was disconnected. I then speculated the recent (yesterday) temperature increase my have increased air pressure in tank, I removed gas cap which resulted in huge gush of air. Does my explanation make sense or is something else going on? The leak seems to have stopped as far as I can tell.
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Buck '88 Coupe, '87 Cab, '88 535i sold, '19 GLC 300 DD Warren Hall, gone but not forgotten |
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03-22-2023, 08:57 AM
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Join Date: Dec 2014
Posts: 14,002
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High pressure fuel system will do that.
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03-22-2023, 11:49 AM
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Registered
Join Date: Nov 2000
Location: Dallas
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Ok thanks...BTW I walked outside after lunch and it was slowly pushing MORE fuel out of the top of the fuel filter. I then removed the gas cap and another big whoosh of air came out. Went back to the fuel filter and no more fuel coming out. Was low 80s today so not surprised the air in tank expanding. will leave cap off till project done.
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Buck '88 Coupe, '87 Cab, '88 535i sold, '19 GLC 300 DD Warren Hall, gone but not forgotten |
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03-22-2023, 04:48 PM
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PCA Member since 1988
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The tank should not build up pressure. It has overflow and breather hoses that route excess fuel and excess vapor to the overflow tank, charcoal canister, and air filter housing or intake. Make sure those are not plugged, or hoses kinked, etc.
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1973.5 911T with RoW 1980 SC CIS stroked to 3.2, 10:1 Mahle Sport p/c's, TBC exhaust ports, M1 cams, SSI's. RSR bushings & adj spring plates, Koni Sports, 21/26mm T-bars, stock swaybars, 16x7 Fuchs w Michelin Pilot Sport A/S 3+, 205/55-16 at all 4 corners. Cars are for driving. If you want art, get something you can hang on the wall! |
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03-23-2023, 12:02 AM
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Join Date: Mar 2019
Posts: 1,634
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The tank WILL build up pressure because of the "Reid Vapor Pressure" of the fuel inside. As the fuel heats up, the pressure will increase. The fuel cap will regulate the vacuum and pressure inside the tank. If you leave the cap on with a fuel hose off, the easiest path for the pressure to go is through the disconnected fuel hose. Leave the cap off and it can't build up any pressure.
The whole system is designed to recover any fuel vapors and store them in the charcoal canister until they can be burned through the engine. |
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03-23-2023, 08:34 AM
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PCA Member since 1988
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I'll defer to you since I'm not as familiar with the diagram for the fuel vapor for the Carreras. My 1973.5 does not do that. The "out" hose from my charcoal canister goes back to the engine and air filter housing. Or rather, it used to.
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1973.5 911T with RoW 1980 SC CIS stroked to 3.2, 10:1 Mahle Sport p/c's, TBC exhaust ports, M1 cams, SSI's. RSR bushings & adj spring plates, Koni Sports, 21/26mm T-bars, stock swaybars, 16x7 Fuchs w Michelin Pilot Sport A/S 3+, 205/55-16 at all 4 corners. Cars are for driving. If you want art, get something you can hang on the wall! |
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03-23-2023, 05:55 PM
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Registered
Join Date: Nov 2000
Location: Dallas
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Just found my own thread… I want to clarify the tank pressure issue since everything is hooked back up and car runs great. I’m confused about tank vent system. I thought the Vapor would be sent back to the engine or the carbon canister. I get a whoosh anytime I open the gas cap which according to Dannobee is correct. Under what situation do fuel vapors from tank get sent to engine or to charcoal canister?
Thanks
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Buck '88 Coupe, '87 Cab, '88 535i sold, '19 GLC 300 DD Warren Hall, gone but not forgotten |
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11-09-2025, 12:10 PM
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